1. Field of the Invention
The present invention relates to an image sensing apparatus which controls exposure, a control method for the image sensing apparatus, a computer program, and a storage medium.
2. Description of the Related Art
Conventionally, auto exposure (AE) control of an image sensing apparatus having a solid-state image sensing element such as a CCD sensor or a CMOS sensor involves appropriately adjusting the exposure conditions, and setting an exposure period according to the sensitivity of the solid-state image sensing element being used when image sensing is performed. When setting the exposure period, all pixel data for one screen is read and divided into a plurality of image blocks, and a specific image block area is extracted from the plurality of image blocks. The luminance levels of the pixel data in the specific image block area are then integrated to derive an integrated value for all of the pixel data in the specific image block area.
The integrated value is compared with a predetermined luminance level, and the exposure conditions are changed if the comparison result shows a difference. The exposure conditions include the charge accumulation period of the solid-state image sensing element, shutter speed, aperture size, and the like. All the pixel data for one screen is again read from the solid-state image sensing element using the set exposure conditions, and the optimal values of the exposure conditions are derived by measuring, comparing and determining the integrated value for the luminance levels of the pixel data in the specific image block area. By repeating the above operations several times, the conditions at which the integrated value is equal to the prescribed luminance level are determined as the exposure conditions (e.g., see Japanese Patent Laid-Open No. 2004-350204, Japanese Patent Laid-Open No. 2002-199272).
However, with the conventional method of AE control described above, aperture control is performed with each accumulation period because of the use of image data from the solid-state image sensing element, resulting in insufficient exposure control response. The possibility also exists of exposure being destabilized due to changes in brightness during the accumulation period. In particular, exposure is controlled intermittently when image sensing is performed at a slow shutter speed because of the longer accumulation period, making it impossible to obtain stable image data due to not being able to respond to changes in the brightness of the object. Note that a slow shutter speed here indicates a shutter speed of less than 1/60 sec.